Looks like turning off Lumen GI did get rid of the flashing. I was really shocked at how different the lighting looks without Lumen, though. Your scene appears to be a gray monochrome? I would go back to my previous advice about making a copy of this level with 100% different and new lighting setup. I don’t know how this Sky Creator works, but something about your lighting here is very strange. Just use the SunSky actor (SunPosition plugin) for basic lighting and see what happens.
Your scene should have the correct colors and overall look without Lumen, and enabling Lumen should just enhance the bounced lighting and AO. Make sure you’re not overly reliant on emissive lighting and trying to use extremely high values there to light the scene. Also, “Surface Cache” in the Ray Lighting Mode is the low quality setting. You probably want to be using “Hit Lighting” for this kind of rendering (this shouldn’t cause flickering though).